In conventional kraft cooking implemented in the 1960-1970-ies in continuous digesters was the total charge of white liquor added to the top of the digester. It soon emerged that the high alkali concentrations established at high cooking temperatures was detrimental for pulp viscosity.
Cooking methods was therefore developed in order to reduce the detrimental high alkali peak concentrations at start of the cook, and thus was split charges of alkali during the cook implemented in cooking methods such as MCC, EMCC, ITC and Lo-Solids cooking.
Other cooking methods was implemented using black liquor impregnation ahead of cooking stages where residual alkali in the black liquor was used to neutralize the wood acidity and to impregnate the chips with alkaline sulfide. One such cooking method sold by Valmet is Compact Cooking where black liquor with relatively high residual alkali level is withdrawn from earlier phases of the cook and charged to a preceding impregnation stage.
One aspect of alkali consumption during the cooking process, i.e. including impregnation, is that a large part of the alkali consumption is due to the initial neutralization of the wood, and as much as 50-75% of the total alkali consumption is occurring during the neutralization and alkali impregnation process. Hence, a lot of alkali is needed to be charged to the initial alkalization. This establish a cumbersome problem as high alkali concentrations had been found to be detrimental for pulp viscosity when charged to top of digesters in conventional cooking. One solution to meet the high alkali consumption and necessity to reduce alkali concentration at start of the cooking process was to charge large volumes of alkali treatment liquors, preferably black liquor having a residual alkali content, but having low alkali concentration, which resulted in presence of relatively large amount of total alkali per kg of wood material but still at low alkali concentration.
IN U.S. Pat. No. 7,270,725 (=EP1458927) Valmet disclosed a pretreatment stage using polysulfide cooking liquor ahead of black liquor treatment. In this process was the polysulfide treatment liquor drained after the pretreatment stage and before starting the black liquor treatment. The polysulfide treatment stage was also preferably kept short with treatment time in the range 2-10 minutes.
In a recent granted US patent, U.S. Pat. No. 7,828,930, International Paper, is shown an example of a kraft cooking process where 100% of the cooking liquor, in form of polysulfide liquor also named as orange liquor, is charged to top of digester and start of an impregnation stage. Here is also the temperature raised from 60° C. to 120° C. at start of the polysulfide treatment stage. However, as shown in example 1 is a liquor to wood ratio of about 3.5 established in the top of the digester by adding a proper amount of water. This order of liquor/wood ratio is often perceived as a standard liquor/wood ratio in continuous cooking necessary for a steady process. According to this proposal is a part of the residual polysulfide treatment liquor at relative high alkali concentration withdrawn and replaced with cooking liquor at relative low alkali concentration at start of the cooking stage, and the withdrawn residual polysulfide treatment liquor is added at later stages of the cook.
In Valmet's recent application WO2013032377 is disclosed a most beneficial method for a polysulfide kraft cooking process. The principles with a low temperature first impregnation stage with polysulfide cooking liquor at low liquor-to-wood ratio in the range 2.0 to 3.2 are disclosed. All the advantages with such conditions are disclosed and are included by reference also to the present invention which fully utilize these conditions. However, the system disclosed in WO2013032377 use a pressurized impregnation vessel preceded by a sluice feeder which may led to higher temperatures in the impregnation vessel for the polysulfide impregnation.
One model to describe cooking conditions is the H-factor. H-factor is a kinetic model for the rate of delignification in kraft pulping. It is a single variable model combining temperature (T) and time (t) and assuming that the delignification is one single reaction. If the activation energy is assumed to correspond to 134 kJ/mol the H-factor could be determined by;H=∫0texp(43.2−16115/T)dT This one single reaction model is described in Gullichsen, Johan; Fogelholm, Carl. Johan (2000), “Chemical Pulping”, Papermaking Science and technology 6A, Tappi Publications, pp. 291-292, and is used throughout the pulping community to define cooking references, and will be used in this patent to define conditions of the cook. There is also an online H-factor calculator, using the single reaction model as outlined above, available at internet at http://www.knowpulp.com/english/demo/english/pulping/cooking/1_process/1_principle/h-tekijan_laskenta.htm, where one could calculate the H-factor for any given stage of the cook, i.e. during heat up (typically during impregnation) as well as during cooking (at full cooking temperature), and the total H-factor established in those stages.
This low H-factor is also disclosed in WO2013032377, and with the H-factor model used as disclosed above, following H-factors apply for respective retention time and temperatures (Time*Temp=H);
60*90=0; 60*100=1; 60*110=3, 60*120=9
90*90=0; 90*100=1; 90*110=5; 90*120=13
120*90=1; 120*100=2; 120*110=6; 120*120=18
Even though slightly different H-factors, or different activation energy than 134 kJ/mol, may apply during the cook, i.e. during initial-, bulk- and final delignification respectively is the same H-factor used for the entire cook, including impregnation and heat up phases for comparative studies, which also is the case in a number of scientific studies published. There are also different H-factors for different wood species, especially between annual plant, hardwood and softwood, but for this patent application is the above identified H-factor, using an activation energy of 134 kJ/mol, used as the base reference for all kinds of wood and all phases of the cook. The H-factor is the best parameter to define process parameters for delignification activity. Hence, an H-factor of 1 is indicating almost no delignification, in cooking processes most often requiring a total H-factor of about 300-1500, and typically about 700 for fully bleached qualities, indicating that only some single digit of percent of total delignification work has been obtained at a H-factor of 1. If a H-factor of only 300 is necessary for the final pulp, as could be the case in high yield cooks, a H-factor of 1 is only indicating that 1/300 of total delignification work is obtained during impregnation, i.e. less than 0.4%.
There has thus been an ongoing development of cooking methods where both alkali concentrations at start of cook is reduced, and increased yield from the cooking process is sought for using among others addition of polysulfide cooking liquor that stabilize the carbohydrates.